you might need to upgrade its motherboard, as the motherboard seems old only being able to support DDR3 ram and a max of 8GB RAM.
If you can get a good motherboard the fastest and most powerful CPU is probably the i9 7980XE.
If your asking in terms of gaming a good cpu is i7 8700k
A budget cpu is the i5 7600k
You may want to check if the PC supports these things